Plastic fibers for improved concrete

ABSTRACT

A concrete article comprised of concrete having therein a reinforcing fiber, where at least about 50 percent of the reinforcing fibers are frayed only at an end or ends of the reinforcing fibers, may be made by mixing concrete, water and a reinforcing fiber for a sufficient time to fray the ends of at least 50 percent of the fibers and curing the mixture to form the concrete article. The fiber may be a reinforcing fiber comprised of at least two filaments bonded together and the filaments being comprised of a polymeric core and a polymeric sheath comprised of a fusing-fraying polymer, such that the reinforcing fiber, when mixed with inorganic particulates, frays predominately only at an end or ends of the fiber.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication Ser. No. 60/214,643, filed Jun. 28, 2000.

FIELD OF THE INVENTION

[0002] The invention relates to plastic fibers for toughening concreteand the concrete containing the fibers.

BACKGROUND OF THE INVENTION

[0003] Generally, concrete is a brittle material with high compressivestrength but low tensile strength. In the concrete industry, allconcrete work is, typically, specified on the basis of the compressivestrength. Any attempt to improve the crack strength (tensile strength)and toughness of the concrete almost always requires the introduction ofreinforcing addition. For example, rebar (steel rods) is added whichprovide structural integrity but does not eliminate cracking. Metal meshhas also been added to reduce cracking but it cannot be used effectivelyto reinforce concrete of complex geometry.

[0004] Plastic fibers have also been used to improve the tensilestrength and toughness (resistance to cracking). However, the additionof synthetic polymer fibers almost always causes a reduction in thecompressive strength. In addition, when plastic fibers are used theytend to only improve either the tensile strength (strength before thefirst crack appears) or the toughness (resistance to cracking), but notboth at the same time.

[0005] Examples of plastic fibers include polypropylene (PP),polyethylene (PE), polyethylene terephthalate (PET), aramids (e.g.,KEVLAR) and polyvinyl alcohol fibers. However, all of these fiberssuffer from one or more problems, such as high cost, low alkalineresistance, low tenacity or low interfacial bonding between the concreteand the fiber.

[0006] Polypropylene and polyethylene have been the most preferred fiberto date due to their high tenacity and low cost. Unfortunately, thesefibers suffer from very low interfacial bonding. To remedy this problem,coatings have been formed on the surface of the fibers by applying aliquid, such as glycerol ether or glycol ether on the fiber surface, asdescribed by WO 980766. Coatings have also been applied by vapordeposition, such as described by JP 60054950. Similarly, chemicallymodifying the surface has been done, such as described by JP 10236855(treatment of the surface of a polyoxyalkylenephenyl ether phosphate andpolyoxalkyl fatty acid ester). Unfortunately, these methods naturallylead to increased cost, complexity and potentially insufficient bondingof the coating to the fiber.

[0007] Another remedy has been the incorporation of inorganic particlesin and on the fiber, such as described by JP 07002554. Unfortunately,the fiber process becomes much more difficult (e.g., fiber breakage) andincreases the cost and, generally, decreases the tenacity of the fiber.

[0008] Further, it is known that larger fibers are preferable forimproving the toughness of the concrete. Unfortunately, larger fibersfurther exacerbate the problem of bonding with the concrete matrixbecause of reduced surface area. In addition, none of these methodsaddress another problem associated with plastic fibers in concrete,which is the tendency of larger fibers to clump together into balls thatare difficult to break up when added to concrete resulting in reducedproperties of the concrete.

[0009] U.S. Pat. No. 5,993,537 and WO 99/46214 describe uncontrolledfibrillation of large fibers in concrete. They describe the desirabilityof fibrillating large fibers into many smaller fibers and partiallyfibrillated fibers. They both describe that fibrillation desirablyshould be so great that the surface area of the fibers increase 50 foldor more. However, this extreme amount of fibrillation may lead toproblems with workability, slumping, mixing and lessen desirabletoughness enhancement of larger fibers.

[0010] Accordingly, it would be desirable to provide an improved fiberfor improving the properties of concrete, for example, that solves oneor more of the problems of the prior art, such as improving thetoughness without increasing the cost of concrete when using inexpensivepolypropylene fibers, while at the same time not create other problems,such as slumping and reduced workability.

SUMMARY OF THE INVENTION

[0011] We have now discovered a new polymeric fiber that has improvedbonding with concrete that results in concrete that has improvedproperties, lower cost or both, compared to other reinforced concrete,which is achieved by a fiber that has controlled fibrillation.

[0012] A first aspect of the invention is a reinforcing fiber comprisedof at least two filaments bonded together and the filaments beingcomprised of a polymeric core, at least partially enveloped by apolymeric sheath comprised of a fusing-fraying polymer that has a lowermelting temperature than the polymer core, such that the reinforcingfiber, when mixed with inorganic particulates, frays predominately onlyat an end or ends of the fiber.

[0013] The reinforcing fiber is comprised of at least two filaments thatare bonded together, such that upon mixing with inorganic particles, thefilaments predominately fray at the ends of the fibers (i.e., frays orfibrillates at the ends) under typical mixing conditions, for example,of concrete. This controlled fraying of the fiber overcomes the problemof inadequate bonding of large diameter fibers by giving greater surfacearea to anchor to at the ends, while not causing a deleterious rise inviscosity when fibers completely fibrillate.

[0014] Herein, predominately fraying at the end or ends means that undertypical mixing conditions encountered when making inorganic curedarticles, such as concrete, a majority of the fibers present in thearticle after curing have not separated into two or more fibers. This isanalagous to the fraying of a rope without the rope splitting into twosmaller ropes.

[0015] A second aspect of the invention is a concrete article comprisedof concrete having therein a reinforcing fiber where at least about 50percent by number of the reinforcing fibers are frayed only at an end orends of the reinforcing fibers.

[0016] A third aspect of the invention is a method of preparing concretecomprised of mixing concrete, water and a reinforcing fiber for asufficient time to fray the ends of at least about 50 percent of thefibers and curing the mixture to form the concrete article. Generally,this amount of fraying results in an increase of surface area of atleast about 2 times to generally at most about 10 times, preferably atmost about 5 times and more preferably at most about 3 times of thesurface area of the original fiber.

[0017] A fourth aspect is a reinforcing fiber comprised of apolypropylene core polymer at least partially enveloped by a sheathcomprised of a fusing/fraying polymer that has a lower meltingtemperature than the polypropylene core and is selected from the groupconsisting of low density polyethylene, maleic anhydride grafted lowdensity polyethylene, ethylene-styrene copolymer or polyethylene havinga melt index from about 5 to about 35 and a density from about 0.9 gramper cc to about 0.965 gram per cc, ethylene acrylic acid copolymer andcombinations thereof.

[0018] The reinforcing fiber may be used in any low temperature curedinorganic article, such as concrete, mortar, gypsum, wall board and thelike. The concrete of this invention may be used in any applicationsuitable for concrete, but it is especially well-suited for parkinggarages, bridge decks, white toppings, tunnels, mining, slopestabilization, architectural purposes, such as landscaping stones, skateboarding rinks, modern architecture, art sculptures, fastsetting/non-slumping ceilings, swimming pools and for repairing andretrofitting existing structures.

DETAILED DESCRIPTION OF THE INVENTION The Reinforcing Fiber

[0019] The reinforcing fiber is comprised of at least two fusedfilaments comprised of a core polymer at least partially enveloped by asheath comprised of a fusing-fraying polymer, such that the fiber, whenmixed with inorganic particulates, frays predominately only at an end orends of the fiber.

[0020] Fraying at an end or ends is when the fiber splits into at leasttwo distinct frayed fibrils, where one end of these fibrils iscompletely detached from the fiber and the other end is still attachedto the fiber. To reiterate, this is similar to a rope fraying at theends. Generally, the frayed fibrils are at most about half the length ofthe fiber prior to being frayed. Preferably, the frayed fibrils are atmost about one third (⅓), more preferably at most two fifths (⅖) andmost preferably at most about one quarter (¼) the length of the fiberprior to being frayed.

[0021] To reiterate, the reinforcing fiber frays predominately at an endor ends when a majority (i.e., greater than 50 percent by number) ofsaid fibers fray only at an end or ends when mixed using a concretemixer having a concrete mix of about 50 to 70 percent by weightaggregate balance Portland cement and a Portland cement to water ratioof about 0.4 to about 0.6 by weight. The amount of fraying may bedetermined by known microscopic techniques. Preferably at least about 60percent, more preferably at least about 75 percent, even more preferablyat least 90 percent and most preferably at least 95 percent of thefibers are frayed only at an end or ends after this mixing. A mostpreferred embodiment is when essentially all of the fibers are frayedonly at the end or ends of the fibers.

[0022] The increase in frayed surface helps to improve toughness of thereinforced concrete. However, when fraying is too extensive toughnessdecreases. In other words, the initial fraying leads to improvement intoughness, but too much fraying/fibrillation is detrimental. Generally,the toughness increases when the surface area of the fiber due tofibrillation increases from at least about 2 times until about 3 times,levels off until about 5 times, slowly decreases from 5 to 10 times andthen drastically decreases above about 10 times the surface area of theoriginal fiber surface area.

[0023] The reinforcing fiber is comprised of at least two filaments thathave been fused together. The reinforcing fiber may be comprised of anyuseful amount of filaments greater than or equal to two. Generally, theamount of filaments that are bonded together is at most about 3000.Preferably, the amount of filaments is at least about 12, morepreferably at least about 36, most preferably at least about 72 topreferably at most about 1000, more preferably at most about 500 andmost preferably at most about 350.

[0024] The reinforcing fiber may be in any shape (i.e., shape of thecross-section), such as round, square, triangular, lobed, star and sheet(i.e., similar to a tape). The filaments may be fused by any suitablemethod, such as fusing the filaments as they are being made by one ofthe processes described below.

[0025] As made, the fibers, generally, are about 0.25 to about 4 incheslong. Preferably, the fibers are at least about 0.6 to at most about 3inches long. Generally, each filament is at least about 0.5 μm(micrometers) to about 1000 μm (micrometers) in cross-sectional area.Preferably, each filament is at least about 1 to at most about 500 μm(micrometers) in cross-sectional area. Generally, the fiber is at leastabout 2 to at most about 2000 μm (micrometers) in cross-sectional area.Preferably, the fiber is at least about 6 to at most about 500 μm(micrometers) in cross-sectional area. More preferably, the fiber is atmost about 100 μm (micrometers) and most preferably at most about 50 μm(micrometers) in cross-sectional area.

[0026] Generally, the core polymer comprises at least about 50 percentby volume to at most about 95 percent by volume of the reinforcingfiber. Preferably, the core polymer comprises at least about 55 percent,more preferably at least about 60 percent and most preferably at leastabout 65 percent to preferably at most about 75 percent.

[0027] The core polymer may be any polymer useful in forming thereinforcing fiber so long as the core polymer is different than apolymer comprising the sheath. A polymer is different when the chemistryis different (e.g., polycarbonate versus polyethylene) or the properties(structure) are different (e.g., branched versus linear polyethylene andhigh density polyethylene versus low density polyethylene).

[0028] For example, the core polymer may be polyolefins, thermoplastichydroxy-functionalized polyether or polyester, polyesters, polyamides,polyethers, polysaccharides, modified polysaccharides ornaturally-occurring fibers or particulate fillers; thermoplasticpolyurethanes, thermoplastic elastomers and glycol-modified copolyester(PETG). Other polymers of the polyester or polyamide-type can also beemployed in the practice of the present invention for preparing thefiber. Such polymers include polyhexamethylene adipamide,polycaprolactone, polyhexamethylene sebacamide, polyethylene2,6-naphthalate and polyethylene 1,5-naphthalate, polytetramethylene1,2-dioxybenzoate and copolymers of ethylene terephthalate and ethyleneisophthalate.

[0029] The thermoplastic hydroxy-functionalized polyether or polyestermay be any suitable kind, such as those known in the art. For example,they can be one of those described by U.S. Pat. Nos. 5,171,820;5,275,853; 5,496,910; 5,149,768 and 3,305,528.

[0030] The polyesters and methods for their preparation are well-knownin the art and reference is made thereto for the purposes of thisinvention. For purposes of illustration and not limitation, reference isparticularly made to pages 1-62 of Volume 12 of the Encyclopedia ofPolymer Science and Engineering, 1988 revision, John Wiley & Sons.

[0031] The polyamides may include the various grades of nylon, such asnylon 6, nylon 6,6 and nylon 12.

[0032] By the term “polyolefin” is meant a polymer or copolymer derivedfrom simple olefin monomers, such as ethylene, propylene, butylene,isoprene, and the like and one or more monomers copolymerizabletherewith. Such polymers (including raw materials, their proportions,polymerization temperatures, catalysts and other conditions) arewell-known in the art and reference is made thereto for the purpose ofthis invention. Additional comonomers, which can be polymerized withethylene, include olefin monomers having from 3 to 12 carbon atoms,ethylenically unsaturated carboxylic acids (both mono- and difunctional)and derivatives of such acids, such as esters (for example, alkylacrylates) and anhydrides. Exemplary monomers, which can be polymerizedwith ethylene, include 1-octene, acrylic acid, methacrylic acid, vinylacetate and maleic anhydride.

[0033] The polyolefins, for example, include polypropylene,polyethylene, and copolymers and blends thereof, as well asethylene-propylene-diene terpolymers. Preferred polyolefins arepolypropylene, linear high density polyethylene (HDPE),heterogeneously-branched linear low density polyethylene (LLDPE), suchas DOWLEX polyethylene resin (a trademark of The Dow Chemical Company),heterogeneously-branched ultra low linear density polyethylene (ULDPE),such as ATTANE ULDPE (a trademark of The Dow Chemical Company);homogeneously-branched, linear ethylene/α-olefin copolymers, such asTAFMER (a trademark of Mitsui Petrochemicals Company Limited) and EXACT(a trademark of Exxon Chemical Company); homogeneously branched,substantially linear ethylene/α-olefin polymers, such as AFFINITY (atrademark of The Dow Chemical Company) and ENGAGE (a trademark of DuPontDow Elastomers L.L.C) polyolefin elastomers, which can be prepared asdisclosed in U.S. Pat. Nos. 5,272,236 and 5,278,272; and high pressure,free radical polymerized ethylene polymers and copolymers, such as lowdensity polyethylene (LDPE), ethylene-acrylic acid (EAA) copolymers,such as PRIMACOR (trademark of The Dow Chemical Company), andethylene-vinyl acetate (EVA) copolymers, such as ESCORENE polymers (atrademark of Exxon Chemical Company), and ELVAX (a trademark of E.I. duPont de Nemours & Co.). The more preferred polyolefins are thehomogeneously-branched linear and substantially linear ethylenecopolymers with a density (measured in accordance with ASTM D-792) of0.85 to 0.99 gram per cm³, a weight average molecular weight to numberaverage molecular weight ratio (Mw/Mn) from 1.5 to 3.0, a measured meltindex (measured in accordance with ASTM D-1238 (190/2.16)) of 0.01 to100 gram per 10 minutes, and an I10/I2 of 6 to 20 (measured inaccordance with ASTM D-1238 (190/10)).

[0034] In general, high density polyethylene (HDPE) has a density of atleast about 0.94 gram per cubic centimeter (gram per cc) (ASTM TestMethod D-1505). HDPE is commonly produced using techniques similar tothe preparation of linear low density polyethylenes. Such techniques aredescribed in U.S. Pat. Nos. 2,825,721; 2,993,876; 3,250,825 and4,204,050. The preferred HDPE employed in the practice of the presentinvention has a density of from 0.94 to 0.99 gram per cc and a meltindex of from 0.01 to 35 grams per 10 minutes, as determined by ASTMTest Method D-1238.

[0035] The polysaccharides, which can be employed in the practice of thepresent invention, are the different starches, celluloses,hemicelluloses, xylanes, gums, pectins and pullulans. Polysaccharidesare known and are described, for example, in the Encyclopedia of PolymerScience and Technology, 2nd edition, 1987. The preferred polysaccharidesare starch and cellulose.

[0036] The modified polysaccharides, which can be employed in thepractice of the present invention, are the esters and ethers ofpolysaccharides, such as, for example, cellulose ethers and celluloseesters, or starch esters and starch ethers. Modified polysaccharides areknown and are described, for example, in the Encyclopedia of PolymerScience and Technology, 2nd edition, 1987.

[0037] The term “starch” as used herein, refers to carbohydrates ofnatural vegetable origin composed mainly of amylose and/or amylopectin,and includes unmodified starches, starches which have been dewatered butnot dried, physically modified starches, such as thermoplastic,gelatinized or cooked starches, starches with a modified acid value (pH)where acid has been added to lower the acid value of a starch to a rangeof from 3 to 6, gelatinized starches, ungelatinized starches,cross-linked starches and disrupted starches (starches which are not inparticulate form). The starches can be in granular, particulate orpowder form. They can be extracted from various plants, such as, forexample, potatoes, rice, tapioca, corn, pea and cereals, such as rye,oats, and wheat.

[0038] Celluloses are known and are described, for example, in theEncyclopedia of Polymer Science and Technology, 2nd edition, 1987.Celluloses are natural carbohydrate high polymers (polysaccharides)consisting of anhydroglucose units joined by an oxygen linkage to formlong molecular chains that are essentially linear. Cellulose can behydrolyzed to form glucose. The degree of polymerization ranges from1000 for wood pulp to 3500 for cotton fiber, giving a molecular weightof from 160,000 to 560,000. Cellulose can be extracted from vegetabletissues (wood, grass, and cotton). Celluloses can be used in the form offibers.

[0039] The sheath at least partially envelops the core polymer.“Partially” generally means that at least about 10 percent of thesurface of the core polymer is covered by the sheath polymer.Preferably, at least about 20 percent, more preferably at least 50percent, even more preferably at least about 75 percent and mostpreferably at least about 90 percent of the surface of the core polymeris covered by the sheath. A preferred embodiment includes a sheath thatessentially covers the entire surface of the core polymer or entirelycovers the core polymer.

[0040] The core polymer preferably is polyolefin, nylon and polyester.More preferably, the core polymer is polypropylene, homopolymer ofpolyethylene, nylon or polyester. Most preferably, the core polymer ispolypropylene having a melt flow rate from about 4 to 20.

[0041] The sheath is comprised of a fusing-fraying polymer (FF polymer).The FF polymer provides for the fusing of filaments as the reinforcingfiber is formed and the controlled fraying at the ends when the fiber ismixed with inorganic particles, such as concrete. The FF polymer mayalso allow the core to be compatible with other sheath components, suchas a mechanical activator polymer described below. “Compatible” meansthat there is sufficient adhesion of the sheath to the core so that uponmixing, for example, in concrete, the fibers do not completely separateunder typical mixing conditions used, for example, in making concrete.It is also believed to help in dispensing and dispersing of the fiberswhen mixed with concrete.

[0042] The FF polymer may be any polymer as long as it has a lowermelting temperature than the core polymer and results in sufficientadherence of the sheath to the core polymer. The melting temperature ofthe FF polymer should be low enough so that during fiber spinning, theFF polymer allows melting of the sheath causing the fusing of filamentswithout substantially affecting the core polymer. This controlled fusingis believed to allow for the controlled fraying when the fibers arelater mixed with inorganic particles, such as concrete. The FF polymermay also provide improved chemical bonding of the fiber, for example, toconcrete.

[0043] Generally, the FF polymer has a melting temperature that is atleast 10° C. lower than the core polymer melt temperature. Preferably,the FF polymer temperature is at least 15, more preferably at leastabout 20 and most preferably at least about 30 to preferably at mostabout 100.

[0044] Illustratively, when the fiber core is polypropylene having amelt flow rate of 12, the FF polymer is desirably polyethylene having amelt index between 5-35 and a density between 0.9 gram per cc to 0.965gram per cc in combination with either an ethylene-styrene copolymer,with a low styrene content or a low density PE (0.870 gram per cc).

[0045] Preferably, the FF polymer is an ethylene styrene copolymer withlow styrene content (below 30 percent of styrene by weight), ethylenestyrene copolymer with high styrene content (above 60 percent of styreneby weight), low density PE (e.g., 0.870 gram per cc), low densitypolyethylene grafted with maleic anhydride (MAH), maleicanhydride-grafted polypropylene, ethylene acrylic acid copolymer (e.g.,PRIMACOR), polyethylene, ethylene-methacrylic acid or combinationsthereof. Most preferably, the FF polymer is ethylene acrylic acidcopolymer. The ethylene acrylic acid copolymer is preferably one that is5-20 percent acrylic acid groups by weight and more preferably 9-14percent by weight acrylic acid groups.

[0046] The sheath may contain other components, such as a mechanicalactivator polymer. The mechanical activator polymer may be used toincrease the surface roughness of the reinforcing fiber, which isbelieved to increase the mechanical bonding of the fiber, for example,to a concrete matrix. The mechanical activator polymer may be employedin any useful amount up to an amount of at most, typically, about 20percent by volume of the sheath. The mechanical activator polymer hasone or more of the following: (i) thermal expansion sufficientlydifferent than the FF polymer, (ii) immiscible with the FF polymer,(iii) solubilized or swelled by water and (iv) displays melt fracturebehavior, such that the surface of the reinforcing fiber is rougher thana fiber formed without the mechanical activator polymer. Examplesinclude nylon, polyvinyl alcohol and thermoplastichydroxy-functionalized polyether or polyester.

Forming the Reinforcing Fiber

[0047] In general, the fibers may be formed by well-known processes,such as melt spinning, wet spinning, or conjugate spinning. The fibersof the present invention may be extruded into any size, or lengthdesired. They may also be extruded into any shape desired, such as, forexample, cylindrical, cross-shaped, trilobal or ribbon-likecross-section.

[0048] The fibers may have the following fiber cross-section structures:

[0049] (1) Side-by-side

[0050] (2) Sheath-core

[0051] (3) Islands-in-the sea and

[0052] (4) Citrus (Segmented pie).

[0053] (1) Side-by-Side

[0054] A method for producing side-by-side bicomponent fibers isdescribed in U.S. Pat. No. 5,093,061, which is incorporated herein byreference. The method comprises: (1) feeding two polymer streams throughorifices separately and converging at substantially the same speed tomerge side-by-side as a combined stream below the face of the spinneret;or (2) feeding two polymer streams separately through orifices, whichconverge at the surface of the spinneret, at substantially the samespeed to merge side-by-side as a combined stream at the surface of thespinneret. In both cases, the velocity of each polymer stream at thepoint of merge is determined by its metering pump speed and the size ofthe orifice. The fiber cross-section has a straight interface betweentwo components.

[0055] Side-by-side fibers are generally used to produce self-crimpingfibers. All commercially available self-crimping fibers are produced byusing a system based on the different shrinkage characteristics of eachcomponent.

[0056] (2) Sheath-Core

[0057] Sheath-core bicomponent fibers are those fibers where one of thecomponents (core) is fully surrounded by a second component (sheath).Adhesion is not always essential for fiber integrity.

[0058] The most common way to produce sheath-core fibers is a techniquein which two polymer liquids (melts) are separately led to a positionvery close to the spinneret orifices and then extruded in sheath-coreform. In the case of concentric fibers, the orifice supplying the “core”polymer is in the center of the spinning orifice outlet and flowconditions of core polymer fluid are strictly controlled to maintain theconcentricity of both components when spinning. Modifications inspinneret orifices enable one to obtain different shapes of core or/andsheath within the fiber cross-section.

[0059] The sheath-core structure is employed when it is desirable forthe surface to have the property of one of the polymers, such as luster,dyeability or stability, while the core may contribute to strength,reduced cost and the like. The sheath-core fibers are used as crimpingfibers and as bonding fibers in the non-woven industry.

[0060] Methods for producing sheath-core bicomponent fibers aredescribed in U.S. Pat. Nos. 3,315,021 and 3,316,336, both of which areincorporated herein by reference.

[0061] (3) Islands-in the-Sea

[0062] Islands-in-the sea fibers are also called matrix-filament fibers,which include heterogeneous bicomponent fibers. A method for producingislands-in-the sea fibers is described in U.S. Pat. No. 4,445,833,incorporated herein by reference. The method comprises injecting streamsof core polymer into sheath polymer streams through small tubes with onetube for each core stream. The combined sheath-core streams convergeinside the spinneret hole and form one island-in-the sea conjugatestream.

[0063] Mixing the different polymer streams with a static mixer in thespinning process also makes island-in-the-sea bicomponent fibers. Thestatic mixer divides and redivides the polymer stream to form a matrixstream with multiple cores. This method for producing island-in-the-seafibers is described in U.S. Pat. No. 4,414,276, which is incorporatedherein by reference.

[0064] The islands-in-the-sea structure is employed when it is desirableto increase the modulus of the fiber, reduce moisture regain, reducedyeability, improve the texturing capability or give the fiber a uniquelustrous appearance.

[0065] (4) Citrus-Type (Segmented Pie)

[0066] The citrus-type bicomponent or segmented pie bicomponent fiberscan be made by polymer distribution and/or spinneret modifications ofthe pack assemblies employed in the methods described above forproducing the side-by-side, sheath-core or islands-in-the-sea fibers.For example, by introducing a first polymer stream and a second polymerstream alternately through eight radial channels toward the spinnerethole instead of two channels, the resultant fiber is an eight-segmentcitrus-type fiber. If the spinneret orifice has the configuration ofthree or four slots on a circle (a common orifice configuration toproduce hollow fibers), the fiber is a hollow citrus-type fiber witheight segments. The hollow citrus-type fiber can also be made by the useof special spinneret orifice configurations with a sheath-core spinpack, as described in U.S. Pat. Nos. 4,246,219 and 4,357,290, both ofwhich are incorporated herein by reference.

The Concrete Article

[0067] A concrete article is comprised of concrete having therein areinforcing fiber, where at least about 50 percent of the reinforcingfiber are frayed only at the ends of the reinforcing fibers. “Frayed” isused in the same way as described above.

[0068] The concrete used to form the concrete article of this inventionmay be any suitable concrete, such as those known in the art. Generally,the concrete is a mixture comprised of Portland cement. Portland cementis used as is commonly understood in the art and defined by Hawley'sCondensed Chemical Dictionary 12^(th) Ed., R. Lewis, Van Nostrand Co.,NY, p. 239, 1993.

[0069] It is understood that the reinforcing fiber in the concrete is asolid at ambient conditions. That is to say, the polymer is added as asolid object and is a solid after the concrete is cured.

[0070] The amount of reinforcing fiber in the concrete generally rangesfrom about 0.05 volume percent to about 10 volume percent of theconcrete article. Preferably, the amount of the reinforcing polymer isat least about 0.1 percent, more preferably at least about 0.3 percentand most preferably at least about 0.5 percent to preferably at mostabout 3 percent, more preferably at most about 2 percent and mostpreferably at most about 1.5 percent by volume of the article.

Forming the Concrete Article

[0071] The concrete article may be made by mixing the reinforcing fiber,water and concrete in any suitable manner. Preferably, the concrete drycomponents (e.g., cement, sand and gravel) are dry mixed first and thenwater is mixed to make a wet mixture. Subsequently, the reinforcingfiber is mixed with the wet mixture for a sufficient time to fray theends of at least 50 percent of the fibers, while not so long that thefibers start to completely fibrillate, for example, such that thesurface area of the fibers increase substantially more than 10 times thesurface area of the original fibers. This mixture is then cast,shotcreted or molded or dispensed by any suitable method, such as thoseknown in the art.

[0072] Generally, the concrete is mixed with the reinforcing fiber forat least about 30 seconds to at most about 20 minutes. Preferably, themixing time is at least about 1 minute, more preferably at least about 2minutes and most preferably at least about 3 minutes to preferably atmost about 15 minutes, more preferably at most about 10 and mostpreferably at most about 5 minutes.

[0073] To the mixture, other additives useful in the formation ofconcrete may be added, such as those known in the art. Examples includesuperplasticizers, water reducers, silica fume, furnace slag, airentrainers, corrosion inhibitors and polymer emulsions.

EXAMPLES

[0074] Examples of Reinforcing Fibers:

[0075] All percents are by weight unless otherwise specified.

Example 1

[0076] A sheath was comprised of 80 percent polyethylene (PE) (density0.913), 10 percent 0.955 PE grafted with maleic anhydride (MAH) and 10percent ethylene-styrene with 30 percent styrene copolymer and a core ofpolypropylene (PP) having a melt flow rate of 12. Each of these wasobtained from The Dow Chemical Company, Midland, Mich.

[0077] The fiber was produced at Hills Inc. (W. Melbourne, Fla.) usingcommercially available melt spinning equipment. A round configurationwas chosen, however, other shapes, such as trilobal, tipped trilobal andcross, micro-denier segmented pie, islands-in-the-sea and striped can beused as well. The sheath/core fibers were fabricated with a ratio ofsheath to core from 25:75 to 40:60 by weight. The conditions used toform the fiber were:

[0078] Extrusion Temperature (° C.) Zone PE Blend Sheath PP Core 1 86215 2 193 240 3 220 250 4 217 260 Melt 266

[0079] Extruder Pressure: 750 psi

[0080] Speed of denier roll: 160 rpm

[0081] Speed of tension roll: 163 rpm

[0082] Draw ratio: 8:1

[0083] Quench temperature: 55° F.

[0084] Temperature of tension roll: 93° C.

[0085] Temperature draw roll #1: 110° C.

[0086] Temperature of draw roll #2: 130° C.

[0087] Spin head temperature: 270° C.

[0088] Optional Spin finish (surfactant): (5656 by GouldstoneTechnologies, Monroe, N.C.) 12 percent by weight in water (applied priorto fiber fusing).

[0089] The reinforcing fiber consisted of 72 fused micro-fiber(filaments) and had an overall denier of about 1800 (72 filaments), atenacity of 4.5 gram per denier and an elongation of 29 percent.

Example 2

[0090] A sheath was comprised of 80 percent PE (density 0.913 gram percc), 10 percent ethylene acrylic acid (PRIMACOR) and 10 percentethylene-styrene with 30 percent styrene copolymer and a core of PPhaving a melt flow rate of 12. Each of these was obtained from The DowChemical Company, Midland, Mich. This reinforcing fiber was made in asimilar manner as Example 1.

Example 3

[0091] A sheath was comprised of 80 percent PE (0.913 gram per cc), 10percent 0.870 PE grafted with MAH and 10 percent PE 0.955 gram per ccgrafted with MAH and a core of PP having a melt flow rate of 20. Each ofthese was obtained from The Dow Chemical Company, Midland, Mich. Thisreinforcing fiber was made in a similar manner as Example 1.

Example 4

[0092] A sheath was comprised of 80 percent PE (0.913 gram per cc) , 10percent ethylene-methacrylic acid copolymer (EMAA), 10 percentethylene-styrene with 30 percent styrene and a core of PP having a meltflow rate of 4. Each of these was obtained from The Dow ChemicalCompany, Midland, Mich. This reinforcing fiber was made in a similarmanner as Example 1.

Example 5

[0093] A sheath was comprised of 70 percent PE (0.913 g/cc), 20 percentethylene acrylic acid, 10 percent ethylene-styrene, with 20-35 percentof styrene copolymer and a core of PP having a melt flow rate of 12.Each of these was obtained from The Dow Chemical Company, Midland, Mich.This reinforcing fiber was made in a similar manner as Example 1.

Example 6

[0094] A sheath was comprised of 80 percent PE (0.913 gram per cc), 10percent ethylene acrylic acid copolymer with 9.6 percent acid groups, 10percent of maleic anhydride grafted low density PE (0.870 gram per cc),and core of PP having a flow rate of 12. Each of these was obtained fromThe Dow Chemical Company, Midand, Mich. This reinforcing fiber was madein a similar manner as Example 1.

Example 7

[0095] A concrete mixture was prepared by blending 12.95 volume percentPortland cement (Holnam Type 1), 35.28 volume percent sand (2NS), 28.9volume percent Pea Gravel, 21.38 volume percent tap water, 0.49 percentsuperplasticizer (assuming 40 percent solids-WRDA-19 from W R Grace) and1.0 volume percent polymer fibers. The ratio of cement to water was0.52.

[0096] The polymer fibers were a bi-component fiber comprised of a coreof polypropylene and a sheath comprised of a PE based blend. Thepolypropylene was a 12 melt flow rate polypropylene (INSPIRE H509-12Gpolypropylene, available from The Dow Chemical Company, Midland, Mich.)and constituted about 70 percent by weight of the fiber.

[0097] The dry ingredients (e.g., cement, sand and gravel) were firstadded and then water was added to make a base mixture. The reinforcingfiber (about 2 inches long) was then added to the base mixture. Thedispensing was easy and fibers did not show any tendency forinterlocking and balling. The total mixing time was about 5 minutes. Theslump measured for this concrete mix was about 120 mm and air contentwas about 6 percent. The fibers fibrillate at the fiber ends enough toincrease the surface area to about 2 times of the original surface areaof the fibers. (For comparison, the slump of a concrete mix containingfibers fibrillated to 5 times is about 75 mm and into individualmonofilaments is about 40 mm.) The concrete mixture containing fiberswas placed into rectangular bar molds that were 4 inches by 4 inches by14 inches and cured in a water bath at a constant temperature of 20° C.for 14 days. In addition, the concrete mixture containing the fibers wascured for 1 and 14 days under the same conditions in cylindrical molds(diameter of 3 inches and height of 6 inches).

[0098] After 14 days of curing, the rectangular bars had an averagefirst crack strength of 4.1 MPa (ranging between 3.9 and 4.6), asdetermined in a 4-point bend test. The toughness was 30-40 Nm using theJapanese Toughness Method JSCE SF4. The compressive strength of thecylindrical bars after 1 and 14 days of curing was 16 MPa and 36 MPa asmeasured by a standard compression test.

Comparative Example 1

[0099] Concrete bars without fibers were made in the same way asdescribed in Example 1. The slump value was about 200 mm. The resultsfor these bars were as follows. The average first crack bend strengthafter 14 days cure was 4.5 MPa. The average toughness was about 1 Nm.The compressive strength of the bars after 1 and 14 days of curing was15 MPa and 42 MPa.

Comparative Example 2

[0100] Concrete bars were made in the same way as in Example 1, exceptthat the fibers were commercially available crimped polypropylene fibersfrom Synthetic Ind. (Synthetic Ind., Chatanooga, Tenn.). The fiber wasabout 2 inches in length, had a cross-section of about 0.6 mm² and atenacity of about 4.5 grams per denier. Fibers dispensed into theconcrete mix without interlocking and no balling action was observed.This fiber does not provide any fibrillation and, as a result, slump isunaffected (200 mm). The average first crack bend strength after the14-day cure was about 3.8 MPa. The toughness was about 25 Nm. Thecompressive strength of the bars after 1 and 14 days of curing was 15MPa and 37 MPa, respectively.

Comparative Example 3

[0101] Concrete bars were made in the same way as in Example 1, exceptthat the fibers were commercial polypropylene fibers from WR Grace Corp(Boston, Mass.). The fiber was about 2 inches in length. The fibershowed a tendency for very strong interlocking and had to beindividually dispensed into the concrete. Slump value of the concretemix was about 125 mm. The fiber showed very little fibrillation (even ifmixing time was increased to 10 and 20 minutes) about 1.3 times of theoriginal surface area. The average first crack bend strength after the14-day cure was about 4.2 MPa. The average toughness was about 28 Nm.The compressive strength of the bars after 1 and 14 days of curing was12 MPa and 35 MPa, respectively.

Example 8

[0102] The fiber of Example 2 was used to prepare several mixesaccording to the method of Example 7. The mixing time of the fiber inthe concrete was varied to achieve different levels of fraying. Theconcrete mixes containing fibers were cast into a 4×4×14 inches mold andcured for 28 days. Toughness was measured by using Japanese ToughnessStandard. TABLE I Frayed Fiber Surface Area Over Original Mix Time FiberSurface Area Toughness Mix (min) (SA_(f)/SA_(o)) (Nm) A 3 1.5 31 B 5 234 C 10 3 36 D 15 5.6 34 E 17 9.5 27 F 20 13.5 21

[0103] The increase in the surface area was measured by counting undermicroscope individual monofilaments that frayed from the original fiberand converting number and length of frayed pieces into surface area.Twenty fibers were measured in every case to calculate average number offrayed monofilaments.

Example 9

[0104] The fiber of Example 5 was used to prepare several batches ofconcrete according to the method of Example 7 and varying the mix timeas in Example 8. TABLE II Frayed Fiber Surface Area Over Original MixTime Fiber Surface Area Toughness Mix (min) (SA_(f)/SA_(o)) (Nm) A 1 1.530 B 3 2 33 C 5 5.8 32 D 7 9.6 27 E 10 16 21 F 15 20 17

[0105] Examples 8 and 9 demonstrate that different sheath polymers havedifferent rates of fibrillation. It also shows that the surface areaincrease, as determined microscopically, may be used as an indicator toshow that the fibers have been fibrillated too much (i.e., the fibersare not predominately frayed at the ends).

What is claimed is:
 1. A reinforcing fiber comprised of at least twofilaments bonded together and the filaments being comprised of apolymeric core at least partially enveloped by a polymeric sheathcomprised of a fusing-fraying polymer that has a lower meltingtemperature than the polymer core, such that the reinforcing fiber, whenmixed with inorganic particulates, frays predominately only at an end orends of the fiber.
 2. The reinforcing fiber of claim 1 wherein at leastabout 60 percent of the reinforcing fibers fray only at the ends whenmixed with concrete and water.
 3. The reinforcing fiber of claim 2wherein the mixing time is at least about 5 minutes to at most about 20minutes.
 4. The reinforcing fiber of claim 1 wherein the core polymer ispolypropylene having a melt flow rate from about 4 to about
 20. 5. Thereinforcing fiber of claim 4 wherein the core polymer is polypropylenehaving a melt flow rate from about 8 to about
 16. 6. The reinforcingfiber of claim 1 wherein the fusing-fraying polymer is low densitypolyethylene, ethylene styrene copolymer, low density polyethylenegrafted with maleic anhydride, maleic anhydride-grafted polypropylene,ethylene acrylic acid copolymer, ethylene-methacrylic acid orcombinations thereof.
 7. The reinforcing fiber of claim 5 wherein thefusing-fraying polymer is ethylene acrylic acid copolymer or ethylenestyrene copolymer.
 8. The reinforcing fiber of claim 6 wherein the FFpolymer is polyethylene having a melt index from about 5 to about 35 anda density from about 0.9 gram per cc to about 0.965 gram per cc orcombinations thereof.
 9. The reinforcing fiber of claim 1 wherein thesheath contains a mechanical activator polymer.
 10. The reinforcingfiber of claim 9 wherein the mechanical activator polymer is nylon,polyvinylalcohol, thermoplastic hydroxy-functionalized polyether orpolyester or combinations thereof.
 11. A reinforcing fiber comprised ofa polypropylene core polymer at least partially enveloped by a sheathcomprised of a fusing/fraying polymer that has a lower meltingtemperature than the polypropylene core and is selected from the groupconsisting of low density polyethylene, maleic anhydride grafted lowdensity polyethylene, ethylene-styrene copolymer, polyethylene having amelt index from about 5 to about 35 and a density from about 0.9 gramper cc to about 0.965 gram per cc, ethylene acrylic acid copolymer andcombinations thereof.
 12. The reinforcing fiber of claim 11 wherein thefusing/fraying polymer is the ethylene acrylic acid copolymer.
 13. Aconcrete article comprised of concrete having therein a reinforcingfiber where at least about 50 percent of the reinforcing fibers arefrayed at an end or ends of the reinforcing fibers.
 14. A concretearticle comprised of concrete having therein the reinforcing fiber ofclaim 1 wherein at least 60 percent of the reinforcing fibers are frayedat an end or ends of the reinforcing fibers.
 15. A concrete articlecomprised of concrete having therein a reinforcing fiber of claim 11wherein at least about 50 percent of the reinforcing fibers are frayedonly at an end or ends of the reinforcing fibers.
 16. The concretearticle of claim 13 wherein at least about 60 percent of the reinforcingfibers are frayed only at an end or ends of the reinforcing fibers. 17.The concrete article of claim 16 wherein at least about 75 percent ofthe reinforcing fibers are frayed only at an end or ends of thereinforcing fibers.
 18. The concrete article of claim 15 wherein atleast about 75 percent of the reinforcing fibers are frayed only at anend or ends of the reinforcing fibers.